1. Field of the Invention
The present invention broadly relates to: (1) holographic drive head and component alignment using a unitized mounting structure; (2) alignment of certain holographic drive head components; and (3) replacement and alignment of the laser in a holographic drive head.
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, holographic memory systems have been suggested as alternatives to conventional memory devices. Holographic memory systems may be designed to record data one bit of information (i.e., bit-wise data storage). See McLeod et al. “Micro-Holographic Multi-Layer Optical Disk Data Storage,” International Symposium on Optical Memory and Optical Data Storage (July 2005). Holographic memory systems may also be designed to record an array of data that may be a 1-dimensional linear array (i.e., a 1×N array, where N is the number linear data bits), or a 2-dimension array commonly referred to as a “page-wise” memory systems. Page-wise memory systems may involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of high and low transparency areas representing data, and the system stores, in three dimensions, the pages of data holographically as patterns of varying refractive index imprinted into a storage medium. See Psaltis et al., “Holographic Memories,” Scientific American, November 1995, where holographic systems are discussed generally, including page-wise memory systems.
In a holographic data storage system, information is recorded by making changes to the physical (e.g., optical) and chemical characteristics of the holographic storage medium. These changes in the holographic medium take place in response to the local intensity of the recording light. That intensity is modulated by the interference between a data-bearing beam (the data beam) and a non-data-bearing beam (the reference beam). The pattern created by the interference of the data beam and the reference beam forms a hologram which may then be recorded in the holographic medium. If the data-bearing beam is encoded by passing the data beam through, for example, a spatial light modulator (SLM), the hologram(s) may be recorded in the holographic medium as an array of light and dark squares or pixels. The holographic medium or at least the recorded portion thereof with these arrays of light and dark pixels may be subsequently illuminated with a reference beam (sometimes referred to as a reconstruction beam) of the same or similar wavelength, phase, etc., so that the recorded data may be read.
Holographic data storage systems often comprise a holographic drive head assembly that is used in recording (writing) holograms to and to reading (reconstructing) holograms from a holographic storage medium. This holographic drive head assembly may comprise a variety of optical, optomechanical, optoelectrical and eletromechanical components. For example, these holographic drive head components may include lasers, beam splitters, lenses and lens arrays (e.g., Fourier Transform focusing/storage lens, expander lenses, relays, scanner lens, etc.), phasemasks, encoders (e.g., spatial light modulators), detector arrays (e.g., cameras), waveplates, filters, mirrors, galvanometers (glavos), etc. Each of these holographic drive head components may be mounted as separate components or may be combined into subassemblies comprising a plurality of components.
The design of such holographic drive head assemblies has traditionally been very complicated and complex. The individual components or subassemblies of such components of the holographic head drive assembly may be mounted on many square feet of a sophisticated optical breadboard. The use of such large optical breadboards may be necessitated by the need to meticulously align the various components using large sized optical test equipment. Because of the extreme sensitivity of the optical alignment of such assemblies, a vibration isolation system may be required for the optical breadboard. Meticulous alignment of the many components and elements of the holographic drive head assembly may also be difficult and time consuming to achieve. In addition, the use of such large optical breadboards may be impractical for commercial holographic storage systems.
Previously, holographic drive head assemblies have also required precise alignment of the detector or sensor array (e.g., camera) for the reconstructed beam in multiple degrees of freedom to attain the necessary relationship with the spatial light modulator (SLM). This alignment procedure may be time consuming and may necessitate complicated hardware to attain the level of adjustment needed or desired. Realignment of this subassembly may also be required if the camera, SLM or both need to be replaced because of failure, malfunction, damage, etc. In addition, reflections, for example, from bonding wires, pads, unused border pixels, etc., on the SLM may result in unwanted noise signals in the holograms that are written on the holographic storage medium which may degrade the overall signal to noise ratio (SNR) of recovered holograms when read or reconstructed. Such reflections may also contribute to the problem of holographic drive head alignment.
Holographic drive head assemblies often use a primary laser, which generates a primary coherent light beam. This primary coherent light beam may be split into a plurality beams (e.g., a data beam and a reference beam) which are used, for example, to record/write holograms to and to read/reconstruct holograms from the storage medium. It may be necessary to replace the primary laser due to laser failure, malfunction, damage, etc. Because the primary laser may be the first optical component in a chain of optical components or subassemblies of the holographic drive head assembly, replacing the primary laser may require the realignment of all of the other components and/or subassemblies of the holographic drive head assembly. This may be a very tedious and time consuming task to achieve. The optical path of the primary laser may also require monitoring of its alignment. The primary coherent light beam generated by the laser may also be susceptible to pointing errors due changes in the ambient temperature, wavelength, electrical current supplied, etc.
Accordingly, what may be needed are ways to: (1) more easily align the various components and/or subassemblies of a holographic drive head assembly; (2) reduce the size of the structure for mounting these various components and/or subassemblies of a holographic drive head assembly; (3) provide a mounting structure for the various components and/or subassemblies which minimizes, reduces, eliminates, etc., vibrations that may affect the optical alignment of the holographic drive head assembly; (4) more easily and precisely align the camera with the SLM, as well as to allow for easy replacement thereof without the need of realignment; (5) minimize, reduce, eliminate, etc., reflections on the SLM; and (6) more easily replace the primary laser without requiring tedious and timing consuming realignment of the other components and/or subassemblies of the holographic drive head assembly.